(1) Field of the Invention
The present invention relates to an error control system which can reliably transmit digital information signals by retransmitting a signal if errors occur in receiving the signal. Particularly, the present invention relates to an error control system capable of transmitting information in real time.
(2) Description of the Prior Art
To reduce the influence of errors in transmitting information through transmission lines, various kinds of error control systems have been proposed. Among them, an automatic repeat request (ARQ) system, which provides high system reliability, is relatively simple to construct, compared with other error correction systems, such as a forward error correction (FEC) system. The ARQ system is, therefore, widely employed in various communication systems, such as satellite communications.
To embody the ARQ system, various systems, such as a G-Back-N system and a selective repeat system, have been proposed. However, these conventional systems have drawbacks in that retransmission frequency increases and throughputs of the systems drastically reduce, when channel conditions are not good. Particularly, the selective repeat system needs a memory for temporarily storing a fixed amount of information on its receiving side to prevent the order of reception signals from changing due to signals retransmitted later on. The system should also be controlled, so that transmission of new information is stopped if the number of retransmission requests increases and exceeds the capacity of the memory. This may further deteriorate the transmission efficiency of the system. In addition, as the retransmission frequency increases, transmission delay, namely, the time period from outputting a signal from an information source to receiving the signal at a destination, inevitably increases.
Accordingly, conventional ARQ systems are useful only for transmitting data which allow relatively large transmission delay, such as computer data. Conventional ARQ systems are hardly applicable for transmitting data such as voice information and image information which is transmitted in real time.
As described in the above, conventional error control systems have drawbacks that transmission delay is increased when the channel condition is not good and retransmission frequency increases. Therefore, the conventional systems are applicable only for the cases allowing relatively large transmission delay caused by retransmission, and not applicable for cases requiring real-time transmission.